Case hardening of ferrous metal parts has been accomplished for many years by way of carbon diffusion processes which utilize and/or produce highly toxic cyanide compounds. A cyanide process is disadvantageous for several reasons; for example, the cost of cyanide disposal treatment is very high due to the toxicity of the material; in addition, the cyanide bath is incompatible with commonly used low temperature quench salts of the nitrate/nitrite type.
Work has been done to provide a commercially acceptable salt bath carburizing process of the non-cyanide type thereby to reduce or eliminate the difficulties attending the use and handling of cyanide compounds; see, for example, Canadian Pat. No. 944,665 "Cyanide Free Carburizing Composition, Apparatus and Process" granted Apr. 2, 1974 to the Park Chemical Company, assignee of the present invention. The Canadian patent discloses a carburizing process in which carbon is diffused into ferrous metal parts by immersion in a bath consisting of a molten chloride/carbonate salt and having a cover of graphite particles.
While the cyanide-free carburizing process of the above-identified prior art was frequently capable of producing satisfactory results, we have determined that the actual carburizing mechanism was not phenomenologically understood. Hence, processes which appeared to be carried out in accordance with the prior art disclosures often produced unacceptable results, and the diagnosis of functional problems was difficult or impossible due to a lack of understanding of the basic process.